This disclosure generally relates to an electrode discharge machining (EDM) apparatus and more particularly to the EDM apparatus having a fixture for securing a workpiece that may be a blade of a gas turbine engine, and method of operation.
A gas turbine engine has several major stages including the compressor, the combustor, and the turbine. Compressed gases leave the compressor and are ignited and burned with the addition of fuel in the combustor. The high pressure combusted gases are then directed through a restricting nozzle and into one or more alternating stages of rotating blades and stationary vanes. The efficiency of the engine, and therefore fuel usage, is greatly dependent upon the nozzle area. Precise and accurate control of nozzle area exiting the combustor is desirable for purposes of maximizing overall engine efficiency and reducing fuel consumption.
To create parts that maximize engine efficiency, electric discharge machining (EDM) is used. EDM is a process in which an electrically conductive metal workpiece is shaped by removing material through melting or vaporization by electrical sparks and arcs. The spark discharge and transient arc are produced by applying controlled pulsed direct current between the workpiece (typically anodic or positively charged) and the tool or electrode (typically the cathode or negatively charged). The end of the electrode and the workpiece are separated by a spark gap generally from about 0.01 millimeters to about 0.50 millimeters, and are immersed in or flooded by a dielectric fluid. The DC voltage enables a spark discharge charge or transient arc to pass between the tool and the workpiece. Each spark and/or arc produces enough heat to melt or vaporize a small quantity of the workpiece, thereby leaving a tiny pit or crater in the work surface. The cutting pattern of the electrode is usually computer numerically controlled (CNC) whereby servomotors control the relative positions of the electrode and workpiece. The servomotors are controlled using relatively complex and often proprietary control algorithms to control the spark discharge and control gap between the tool and workpiece. By immersing the electrode and the workpiece in the dielectric fluid, a plasma channel can be established between the tool and workpiece to initiate the spark discharge. The dielectric fluid also keeps the machined area cooled and removes the machining debris. An EDM apparatus typically includes one or more electrodes for conducting electrical discharges between the tool and the workpiece.
During airfoil manufacture, the trailing edge of the airfoil is machined. More specifically, an electric discharge machining process is used to shape the airfoil trailing edge into the desired dimensions. Electrical discharge machining is a well known process for forming features, such as holes, slots and notches of various shapes and configurations, in an electrically conductive workpiece. Conventional EDM apparatuses typically employ an electrode, having a specific shape, that is advanced toward the workpiece. A suitable power supply is applied to create an electrical potential between the workpiece and electrode for forming a controlled spark which melts and vaporizes the workpiece material to form the desired feature. The cutting pattern of the electrode is usually computer numerically controlled (CNC) whereby servomotors control the relative positions of the electrode and workpiece. During machining, the electrode and workpiece are immersed in a dielectric fluid, which provides insulation against premature spark discharge, cools the machined area, and flushes away the removed material.
At least some known EDM fixtures are used to machine turbine airfoil assemblies. Because of a curvature of the airfoil, accurately forming features on the airfoil, including the openings in the airfoil and the profile of trailing edge, are a time consuming and difficult task. To facilitate improving the EDM process, a clamping apparatus is used to secure the workpiece. The consistent, repeatable locating and securing of blade assemblies facilitates precision machining. Most known clamping assemblies or fixtures are complex and expensive, and do not assure a finished workpiece that meets specifications. Typically, a fixture relies on a specific point on the platform adjacent the airfoil to locate the workpiece within the fixture. Due to the curvature of the airfoil and location of the clamping or securing mechanism(s), this locating datum on the platform may not create consistent, uniform parts from workpieces placed within the fixture.